1. Field of the Invention
The present invention relates generally to an oscillator for generating signals having various frequencies for use by digital circuits and more particularly to an oscillator located internally as part of a semiconductor device for generating signals of various frequencies for use by remaining circuits of the semiconductor device.
2. Description of the Prior Art
Oscillators are circuits that generate signal(s) having a given frequency for use generally by digital circuits and systems. These generated signals may be system clocks employed by digital circuits for causing the development of other signals and for use in causing synchronization and/or alignment of various signals with respect to other signals and for other like usages. Currently, for example, oscillators generate a system clock using either a crystal oscillator or a resistor-capacitor (RC) relaxation oscillator as a reference.
In crystal oscillators, a quartz crystal device (crystals are cleaved from natural substances that are used in such applications) is utilized as a reference for generating a signal having a given frequency (or oscillations). This signal is buffered prior to being used as a system clock. Use of a crystal causes the frequency of the clock to be constant. This is important because system clock signals, in general, are expected to be "clean" thereby ensuring accurate results when used by various circuits to generate other digital signals. In essence, a system clock signal is used to direct traffic in a synchronized fashion in a circuit or a device. This is in addition to generating other types of digital signals as just mentioned.
However, due to a multitude of reasons, crystals are relatively expensive in addition to being bulky in size and are thus undesirable. Furthermore, a crystal device commonly resides on the outside of, for example, a semiconductor device thereby increasing the size of an electronic board, card or system on which the semiconductor device resides. In this connection, a crystal device, being located externally to a semiconductor device, adds to the complexity of laying out the board (designing where the signals on the board should be routed) on which the crystal device resides. These issues ultimately cause the costs associated with the manufacturing of the board, card or system, which includes the crystal to increase. Moreover, crystal oscillators are difficult to power down, in part, because they require several milliseconds of oscillation frequency prior to stabilization of the frequency. On the other hand, if they are left running, they continue to burn power thereby adding to power consumption.
In an effort to reduce power consumption when using crystal oscillators, designers (or users of crystal oscillators) employ crystal oscillators having much lower frequencies than that which is desired. For instance, frequency that is generated by a crystal oscillator can be on the order of 50 to 200 times less than the desired frequency. The ultimate desired frequency is synthesized using the crystal-generated frequency as reference. Synthesizing requires complicated design and circuitry. Additionally, the desired frequency is fixed and difficult to tune (or adjust) to other frequencies that may actually be more desirable.
The RC relaxation oscillator includes an RC circuit for generating a sawtooth-shaped oscillation. This is employed as the basis for synthesis to obtain the desired frequency. This type of oscillator is similar in operation to crystal oscillators, with the exception that RC oscillators are generally less expensive and less bulky in size than crystal oscillators.
However, while the frequency generated by the RC oscillator can be adjusted by changing the value of the resistor or the capacitor, which are an integral part of the oscillator, one of the drawbacks of this type of oscillator is that it is not as accurate as the crystal oscillator. In fact, variations in temperature, process and supply voltage affect the frequency of oscillation more than that which is desired or may be tolerated at times. Further drawbacks to using an RC oscillator include taking several milliseconds for the synthesizer to reach the desired frequency after the oscillator is activated (this can be reduced by varying the design of the oscillator but any such change to a design is time-consuming and cumbersome). Additionally, this type of oscillator resides on the outside of a semiconductor device.
Therefore, the need arises for an oscillator circuit that resides internally to a semiconductor device, causes generation of signal(s) having accurate frequencies for use by digital signals and circuits, and prevents changes to the frequency of the generated signal that may be due to: variations in the process employed for manufacturing the semiconductor device; temperature variations; or power supply voltage variations.